Rob Hall gives an update on two important diseases, both of which are spread by biting midges and are currently causing issues in the UK.

Bluetongue Virus

Bluetongue is an infectious viral disease, which was first detected in the 1700s. It is widely found in many parts of North Africa and the Middle East, and there have been occasional outbreaks of the disease recorded in Southern Europe over the past century. In 2006 the first major outbreak was seen in Northern Europe, followed by re-emergence in 2015 and again in 2023.
The Bluetongue virus (BTV) can infect all ruminants, including cattle, sheep and deer. Sheep show the most severe illness, whereas cattle and deer show milder signs of the disease but act as a reservoir for re-infection. BTV is spread mainly by Culicoides biting midges, but other biting insects may also transmit the virus.
There are 29 different strains (‘serotypes’) of the virus, each of which has a different version of the virus protein 2 (VP2) on its surface. The strains vary in the severity of disease which they cause: BTV-10 killed 75% of the sheep it infected in 1956, whereas the BTV-8 strain in 2006 and 2015 caused 25% mortality. There is also evidence that BTV-8 specifically can be transferred from a dam to her foetus during pregnancy, as well as being spread by contaminated objects such as needles, and can persist in semen (including frozen).
The current disease situation, which is thought to have been introduced to the UK by midges blown across from Europe, is caused by BTV-3. There have now been over 56 blood test positive cases on 33 premises in Norfolk and Kent (as of 25th January 2024), but no clinically affected animals have yet been found in the UK.

An early case of Bluetongue showing lesions around the mouth and depression [photo NADIS]

Clinical signs:

  • Fever
  • Redness of the mucus membranes
  • Sores on the nose, gum and inside the mouth
  • Swelling of the face, lips and tongue
  • Lameness
  • Abortion or deformities in lambs or calves
  • Lethargy
  • Milk Drop
  • Death
  • The “blue tongue”, from which the disease gets is name, is not frequently seen. Animals may also develop breathing difficulties if the tongue swells.

Oral lesions in a Bluetongue infected cow [photo Belal Hossain]

This strain causes death of around 30% of the sheep it infects. There are no treatments, and no effective vaccines exist for BTV-3 (unlikely the BTV-8 strain in 2006 which was controlled through vaccination). Sadly, there is no cross-protection from the existing vaccine.
In the current cold weather, the midges’ lifespans are shorter and they are not reproducing as quickly. There is a delay between a midge ingesting the virus and the virus being shed in their salivary glands. This delay is temperature dependent, and only a small percentage of midges become infectious. As a result, most midges which pick up the virus do not live long enough to pass the disease to the next animal they bite. It is hoped that a combination of a cold winter and culling of infected ruminants will keep the spread of the disease low while a vaccine is developed.
Bluetongue disease is notifiable, and so suspicion of the disease must be reported to APHA / Defra, with affected animals humanely culled to limit the risk of further spread. Control zones are also put in place with blood sampling and movement restrictions. Purchasing of animals from the south east of England or areas of Europe where the disease is active is not recommended. You must call your vet practice immediately if you suspect Bluetongue in your animals!

Schmallenberg Virus

In late summer of 2011, adult cattle herds in the Netherlands and Germany experienced outbreaks of disease including mild or moderate fever, reduced milk yield, lower feed intakes, loss of body condition and diarrhoea. None of the common causes of scour were found when testing was performed.
By December, cases of abortion and stillbirth, often with fetal abnormalities, were seen. These were mainly sheep, but also cattle and goats across the Netherlands, Germany and Belgium. A novel virus was found to be the cause of the diseases, which was named ‘Schmallenberg virus’ (SBV) after the German town where the virus was first identified.
Schmallenberg, like Bluetongue Virus, is transmitted by midges of the Culicoides species, and does not spread directly from animal to animal. The disease affects sheep, cattle and goats. Although it has been detected in other ruminants like deer and llama, the virus is not associated with disease in these animals and their role in its spread is unknown. The rate of spread is closely linked to the number of midges, peaking in late summer / early autumn and drop sharply once frosts begin.
Its spread is closely linked to the numbers of midges, which typically peak in late summer/early autumn (August/September) and drop sharply once frosts begin. Some transmission does occur over winter when midge numbers are low, but it is very reduced. Unlike Bluetongue (which cannot be passed between midges) there is some evidence that Schmallenberg virus can pass from a female midge to her offspring, which may help to explain why the disease doesn’t disappear entirely. Some bulls excrete virus in their semen after infection (it is not known how long for) though sexual transmission has not been demonstrated. Schmallenberg virus has not been found in ram or buck semen so far.

Culicoides midges are responsible for the transmission of Bluetongue and Schmallenberg viruses [photo Ed T. Schmidtmann, USDA/ARS]

During the initial 2012/13 outbreak, infected midges were blown across from Europe into the eastern counties. LLM’s bulk tank surveillance and blood testing at the time showed presence of the disease on all the farms we monitored, with APHA finding 25-7% of animals had antibodies against the disease (varying by county). The impact of the disease varied, with an average of 3% additional lamb mortality in positive flocks, however some flocks experienced 50-60% lamb losses.
SBV is similar to other members of the Orthobunyavirus group such as Akabane virus, which circulate in Asia, Africa and Australia. These diseases are seen in cycles of 3-5 years: there is no re-emergence of disease in the few years after an outbreak while ‘herd immunity’ is high. As immunity in an area falls, including as immune animals are culled and naïve young stock replace them, the risk of the disease re-appearing increases. Disease is more likely in warm years with high midge numbers. By 2016/17, conditions allowed a widespread re-emergence of the virus with associated lamb losses and deformed calves.
We’ve been closely tracking the situation using our bulk tank surveillance scheme (Infectious Disease Check), and saw widespread exposure of dairy herds across our entire practice range in September/October 2023 followed by further increases by December/January. We have also detected individual animals testing positive for antibodies (signalling exposure), and early-lambing flocks are already seeing increased rates of losses in their lambs associated with Schmallenberg disease. We’re anticipating problems with deformed calves from now on as we go into spring calving so bear this in mind if you are struggling to sort out a difficult calving.

Seroconversion has been seen in our surveillance dairies from Q4 2023

Clinical Signs:

  • Outbreaks usually last 2-3 weeks on a farm, with individual animals only affected for a few days
  • Acute disease in adult cattle includes fever, reduced milk yield, lower intakes, loss of body condition and diarrhoea
  • Adult sheep and goats generally do not show signs of clinical disease
  • The biggest issue is fetal deformities – the virus damages developing nerve tissue, leading to brain and spinal cord issues.
  • As a result, sheep, goats and cattle can give birth to live or dead offspring with muscle / skeletal deformities such as bent limbs and fixed joints. This can affect some or all joints, and the spine can also be affected.
  • Persistent flexion/fusion of the joints (arthrogryposis or “contracted tendons”) is the most commonly observed birth defect
  • Brain defects can cause ‘dummy’ animals – blind, poor balance, unable to stand or suck, possibly with seizures
  • The susceptible stages of pregnancy for fetal deformities are 62-180 days in cattle and 25-50 in sheep (older fetuses can clear the virus themselves)
  • In sheep or goats, only one of a pair of twins can be affected
  • Deformities lead to increased difficulty of lambing / calving – caesareans are more likely to be required to avoid injury to the dam!

Persistent flexion/fusion of the joints (arthrogryposis or “contracted tendons”) is the most commonly observed birth defect
Brain defects can cause ‘dummy’ animals – blind, poor balance, unable to stand or suck, possibly with seizures
The susceptible stages of pregnancy for fetal deformities are 62-180 days in cattle and 25-50 in sheep (older fetuses can clear the virus themselves)
In sheep or goats, only one of a pair of twins can be affected
Deformities lead to increased difficulty of lambing / calving – caesareans are more likely to be required to avoid injury to the dam!